Purchasing BU Definition - Receipt Date Exchange Rate - How is it used

There is the option on the purchasing business unit to have the Receipt date be the driver for determining the exchange rate when adding receipts. However it doesn't seem that these amounts are ever used in any variance. For Standard costed items the putaway(20) transaction is still put away at standard with a PPV (400) being created for the difference in base currency between PO and standard cost. Even the ERV (402) calculation uses the difference between the PO and invoice.   So my questions are:  - Why is the is option available  - Are there other variances the use the